client: Add detailed docstrings to PdfRestFilesClient and AsyncPdfRestFilesClient

- Added Google-style docstrings to all methods in `PdfRestFilesClient` and
  `AsyncPdfRestFilesClient`, documenting parameters, return types, and
  exceptions for improved clarity and usability.
- This change enhances developer experience by providing comprehensive
  method documentation directly in the codebase.

Assisted-by: Codex

Author: datalogics-kam